1. Field of the Invention
Embodiments of the present invention generally relate to a voltage controlled oscillator having reduced phase noise.
2. Description of the Related Art
A voltage controlled oscillator (VCO) is an electronic circuit that is used to generate an electrical signal that oscillates at a frequency which is controlled by an input voltage. In other words, the input voltage to the VCO sets the frequency of the oscillating VCO output signal. As the input voltage is varied, the VCO's output signal frequency changes in accordance to well-known equations. This VCO voltage-frequency relationship is critical for many different communication applications. Consequently, one or more VCOs are found in virtually every modern communication device, such as in cell phones, radio transmitters and receivers, satellite receivers, GPS systems, wireless data systems, etc.
FIG. 1 shows a circuit diagram of a typical VCO. The VCO includes four transistors 101-104, an inductor 105, and a capacitor 106. The inductor 105 in parallel with capacitor 106, forms a classic resonant or tuned circuit, also commonly referred to as an LC “tank” circuit 107. More particularly, the LC tank circuit 107 is defined as a second-order circuit because its voltage or current can be described by a second-order differential equation. FIG. 2 is a plot depicting the AC analysis results of a second-order LC tank circuit that may be stimulated by a unit current source. It can be seen that the second-order LC tank circuit has a single resonant frequency, w, which is referred to as being a first or “fundamental” harmonic. This second-order LC tank circuit is used in conjunction with the four cross-coupled transistors 101-104, to generate an oscillating, sinusoidal waveform as shown in FIG. 3. The oscillating, sinusoidal waveform 301, output from the VCO, serves as a reliable, constant reference signal that other circuits rely upon to perform their respective functions.
Ideally, the VCO output signal should have perfect periodicity. This is characterized by each cycle of the sinusoidal waveform having the same period or duration as all the other cycles. For example, referring to waveform 301 of FIG. 3, t0-t1 should be the same time as t1-t2; t1-t2 should be the same time as t2-t3; and t2-t3 should be the same time as t3-t4; etc., (t0-tn represent the zero-crossings where the signal crosses a reference voltage). Unfortunately, noise is always present and poses a serious problem to any circuit design. Unavoidably, the transistors and other elements may introduce a rather significant amount of noise as part of the VCO functionality. The undesired effect is that individual zero-crossings can deviate from the ideal condition (e.g., the zero crossings may sometimes occur earlier, while at other times, they may occur later than expected). The changes in the times when the VCO waveform crosses the reference voltage, is referred to as jitter. In terms of frequency, the deviations result in variable changes to the VCO's frequency during its normal course of operation. The changes in frequency is commonly referred to as phase noise.
Phase noise is highly undesirable because it detrimentally impacts the overall performance of the system. A standard measurement of the performance of a system is its signal-to-noise ratio. The signal-to-noise ratio is defined as the ratio of a signal power to the noise power corrupting the signal. Consequently, the phase noise attributed to the VCO directly reduces the system's signal-to-noise ratio. In real terms, a lower signal-to-noise ratio translates into degraded signal quality and/or shorter range of coverage. Thus, it is an important design criteria to reduce or minimize the phase noise inherent to VCO circuits in order to attain a high degree of signal-to-noise ratio (SNR).
One way to potentially reduce a VCO's phase noise entails adding additional circuitry with active components to somehow cancel or compensate for the phase noise. However, this solution consumes additional power. For portable applications (e.g., cell phones), this approach may impose further drains upon battery life. Furthermore, the additional circuitry adds to the cost of production and may only ameliorate the phase noise by a relatively small margin.
Presently, designers are faced with accommodating the phase noise associated with VCOs and the attendant lower signal-to-noise ratio versus trading off cost/power in terms of adding circuitry to achieve a lower degree of VCO phase noise.